The clinical and pathologic features of the toxicosis produced by nomensin, a sodium-selective carboxylic ionophore, were characterized in swine. Clinical signs of toxicosis consisted of lethargy, anorexia, diarrhea and muscle weakness. Elevation of serum creatine phosphokinase and aspartate aminotransferase activity coincided with the development of necrosis in skeletal and cardiac muscle. In the latter, lesions were limited to atrial myocardium. The reason for this selective localization of the cardiac lesions in this animal species is unknown.